Background and Purpose: The premenstrual syndrome is the emergence of a set of symptoms before menstrual which results in imbalance in individual’s life and daily activities. Today the prescription of herbals is recommended for the treatment of the illnesses. The present study aimto pinpoint the effect of primrose oil on the premenstrual syndrome. Methods: The study was a triple blind clinical-trial which was conducted with 80 female university students residing in Lorestan Medical University hostels. The samples were selected based on the DSMIV-TR criterion for the premenstrual syndrome. Using block-randomization method, they were categorized into two groups (each 40), one treated with primrose oil and the other with placebo. The dose of the primrose oil is 1500 mg per day for the first group. The total score for sever symptom (for each from 0 to 10) was assigned at first monthly and then for three months. For the data analysis &chi;2 and repeated measures were deployed. Results: The score for symptom severity before the intervention for the primrose group was at the beginning. At the end of the third month, it got therewas a significant difference in the reduction of symptom severity in the follow-up periods . In the Placebo group, the score for sever it ysymptom before the intervention was . At the end of the third month after treatment, it was This didn’t show any significant difference in the decrease of the symptom severity in the follow-up period. There was a significant difference for the symptom severity in both primrose and Placebo group . Conclusion: Taking the effect of primrose oil on premenstrual syndrome reduction and the harmlessness of hessrb medicals into account,bioresearches recommend primrose oil for the treatment of premenstrual syndrome.